1. Field of the Invention
The present invention relates to an image processing method which converts R (red), G (green) and B (blue) multivalued image signals into color signals corresponding to coloring materials used for image formation, an image forming method which forms an image on the basis of the color signals converted by the image processing method, computer programs which execute the image processing method and the image forming method, a storage medium which stores the computer programs.
2. Related Background Art
A color ink-jet printer which is adopted as an example of a color output apparatus often represents an image by C (cyan), M (magenta) and Y (yellow) three colors or C, M, Y and K (black) four colors.
Ordinarily, in a printer, image data to be printed is converted into data for color inks of the printer on the basis of predetermined color processing parameters, and then the converted image data is actually printed. The color processing parameters respectively correspond to different modes such as a coloring material used for the image formation, a recording material on which the image is to be formed, an image forming method and the like, and these parameters are adjusted to be able to obtain an optimum image in each mode.
Hereinafter, as a method of representing colors, a method based on a color system of CIE1976 L*a*b* uniform color space will be explained.
A process of converting former image data represented by R, G and B signals into C, M and Y data capable of being printed by the printer includes a process called “color space compression (or color gamut mapping)”. This is the process by which the colors capable of being reproduced by additive color mixture represented by R, G and B come to correspond to the colors reproducible by the printer. Generally, a color reproduction range (or color gamut) is different for each device, and, particularly, the color reproduction range of the printer is remarkably different from that of a monitor. Ordinarily, the color reproduction range to be displayed by R, G and B on the monitor or the like is wide, whereby a conversion expression such as sRGB or the like suitable for the characteristic of the monitor is used to calculate the colors (L*a*b*) represented by the monitor. On the other hand, the color reproduction range of the printer depends on the coloring materials of the printer and a recording medium on which the image should be formed (hereinafter, this medium is called “image-formed recording medium” for convenience of explanation). FIG. 5 is a pattern diagram showing the color ranges reproducible by the monitor and the printer. As can be understood from FIG. 5, many colors can be reproduced by the monitor but can not be reproduced by the printer. For this reason, the operation to correlate the colors reproduced by the monitor with the colors reproducible by the printer.
FIG. 6 is a schematic diagram showing the section obtained by cutting the color reproduction range based on one hue. In FIG. 6, the axis of ordinate indicates brightness L*, i.e., an achromatic color line from white to black (hereinafter called a gray line), the axis of abscissas indicates saturation (or chroma) calculated by √{square root over ((a*{circumflex over ( )})}2+b*^2), the broken line indicates the data of the monitor, the solid line indicates the data of the printer, and the heavy line indicates the data after the color space compression. Although there are conventionally various known color space compression methods, the method of deforming and compressing an entire image by mapping maximum saturation points as shown in FIG. 6 is commonly used. That is, for example, a color X reproducible by the monitor but unreproducible by the printer is mapped into a color X′ reproducible by both the monitor and the printer. By this method, since gradation capability can be satisfactorily maintained, whereby it is suitable particularly to reproduce a photographic image.
In a photograph, so-called “memory colors” such as a color of sky, a color of green, a color of skin (called a skin color hereinafter), and the like are important, and particularly the skin color is most important. That is, the skin color is very delicate as well as a tint and saturation. Thus, if the skin color is yellowish, too reddish or bluish even a little, this photograph makes a seriously bad impression on a viewer.
As a method of reproducing a desirable skin color, a method of mapping only a specific skin color area into a desirable skin color area after the color space compression is performed is known. However, if the skin color is reproduced in this method, a discontinuous portion appears in the vicinity of the skin color area, whereby there is a problem that a pseudo contour or the like appears in the reproduced image. Moreover, a complicated operation is necessary to reproduce the desirable skin color, whereby there is a problem that it imposes a load (or burden) on a computer used for image formation.
Incidentally, a recording apparatus can generally perform recording on various recording media. For example, the ink-jet printer can perform the recording on a plain paper, a coated paper specific to ink-jet printing, a glossy paper, and the like. Here, it should be noted that the colors of these recording media themselves are frequently different from others, whereby it is difficult to reproduce almost the same or equivalent color on the different recording media. Particularly, it is difficult to reproduce the same light color (or pale color) easily influenced by the inherent color of the paper itself, on the different recording media.
Incidentally, the L axis, i.e., the gradation (gray line) from white to black, does not change during the color space compression, whereby it is necessary to adjust from the first the gray line so that there is visually neither jump of gradation nor fluctuation of hue.
FIG. 7A is a graph showing discharge amounts of C, M and Y inks for eight-bit output values 0 to 255 (white to black) in gray scale, and FIG. 7B is a graph showing a state that colorimetry values of 17 gradations having same intervals and selected at this time from 256 gradations in the gray scale are smoothly connected with others. It should be noted that the 17 gradations include R=G=B=0 (black point, output 255) and R=G=B=255 (white point, output 0). The colors in the gray scale close to black are substantially gathered together in the vicinity of (a, b)=(−3, −8). This numeric value (a, b)=(−3, −8) represents the desirable color of gray found by the inventor in a visual experiment.
However, as can be understood from FIG. 7B, in the vicinity of white, the color does not necessarily close to the above numeric value, because the image-formed recording medium itself has the color of (a, b)=(0, −4). Thus, since a light-color area where the coloring materials to be recorded are few is strongly influenced by the color of the image-formed recording medium itself, if it intends to numerically conform the color in the vicinity of white to (a, b)=(−3, −8), it is observed that the color skips suddenly, whereby gradation capability deteriorates.
As can be understood from FIG. 6, if the color having intermediate saturation is subjected to the color space compression, then the saturation of the compressed color decreases considerably, whereby this color comes to close to the L axis (gray line). Such colors close to the gray line are greatly influenced by the color of gray line, and include many kinds of skin colors that brightness is high and saturation is low (i.e., the skin colors close to the gray line).
In the above conventional technique, it is not recognized that the reproduction of the gray line influences the reproduction of the skin color, whereby the color processing parameters for converting the gray line are adjusted without considering the skin color absolutely. Therefore, the color processing parameter of the color of light gray (or pale gray) is conventionally adjusted visually by entirely considering the connection of the gradations of gray, that is, without any concrete numerical target. Moreover, it is conventionally difficult to conform the color of the image formed on the image-formed recording medium having one color with the color of the image formed on the image-formed recording medium having other color.